Metal films are utilized in semiconductor technology to wire together various components formed on a semiconductor wafer. Metal in semiconductor processing can also be used to function as gate electrodes in MOS structures, and as electrodes in thin film capacitors. Elemental aluminum and its alloys have been the traditional metals utilized. Advantages of aluminum include its low resistivity, superior adhesion to SiO.sub.2, ease of patterning, and high purity.
However, aluminum is not without drawbacks. First, the electrical contact of aluminum with silicon while adequate for large scale integration (LSI) level technology, has reached its limit in the very large scale integration (VLSI) era. Another drawback associated with aluminum is electromigration. This is a phenomenon that occurs in aluminum metal leads while the circuit is in operation, as opposed to a failure occurring during fabrication. Electromigration is caused by the diffusion of the aluminum in the electric field set up in the lead while the circuit is in operation. It is also enhanced by thermal gradients that arise in the lead from the heat generated by the flowing current. The metal thins and eventually separates completely, causing an opening in the circuit. Electromigration occurs over time and may not show up until a device has had many hours of operation. This problem is presently overcome by designing wide overlap regions at contacts, using a constant film thickness, or alloying aluminum with other materials such as copper or silicon.
Electromigration becomes more of a worry as the level of integration increases. The higher number of circuit components in VLSI, ultra large scale integration (ULSI) and beyond creates more current flow and generates more heat. Accordingly, as integrated circuit patterning schemes continue to miniaturize into to submicron dimensions, aluminum-based metallurgies will become increasingly marginal for handling the increased circuit speed and current density requirements.
Elemental copper or its alloys would be an attractive alternative as the primary conductor in VLSI and ULSI multilevel metallization systems. Copper has an even lower resistivity than aluminum, and significantly higher electromigration resistance. However, a primary problem with integrating copper metal into multilevel metallization systems is the difficulty of patterning the metal using etching techniques. For devices of submicron minimum feature size, wet etch techniques for copper patterning have not been acceptable due to, a) liquid surface tension, b) isotropic etch profile, and c) difficulty in over-etch control.
Several promising methods for producing patterned predominately Cu interconnects have been proposed, including selective electroless plating, selective chemical vapor depositing, and high temperature reactive ion etching. Lift off processing is also another alternative for patterning copper. One alternate technique of metal wiring of copper, and in accordance with an aspect of the invention, would comprise the patterning and etching of a trough and/or contact within a thick layer of insulating material such as SiO.sub.2. Thereafter, a thin layer of a barrier metal, such as Ti, TiW or TiN, is provided atop the insulating layer and within the trough and/or contact. Such functions as a diffusion barrier to prevent inter-diffusion between the metal to be subsequently deposited and silicon, and between such metal and oxide. After barrier metal deposition, a layer of metal is deposited to completely fill the trough. With troughs, which would be created prior to metal deposition, a desired metal pattern is defined such that a planar metal removing technique down to the surface of the insulating layer will leave remaining desired patterned electrically conductive metal lines.
One such planarizing technique is chemical mechanical polishing. However to date, chemical mechanical polishing of copper and it alloys has not been well understood or developed. Accordingly, a need remains for improved chemical mechanical polishing techniques and chemical mechanical polishing slurries for copper and its alloys.